I've been cut off by slow moving vehicles too often to want to see dual speed limits. Whether oblivious, or deliberate, there will always be people that pull into the passing lane doing dead-on, or less than, the limit and, unlike the Autobahn, they're not at fault if you plow into the back of them (now if they side swipe you, that's a different story - but either way, if you're in a sports car from the '70s, the safety features won't be the same as the late model crown victoria you've just tangoed with, so 'fault' may only matter to the estate of one of the drivers).
It's about Awareness, and Courtesy, but people need these things naturally, legislating either just adds more 'fuzzy' laws to the books.